The present invention relates to a chuck for gripping a cutting tool such as a drill, an end mill or the like, and more particularly to such a chuck incorporating needle rollers.
With respect to the above-mentioned chuck, a needle roller type of chuck has been already known as apparant from the disclosure in Japanese Utility Model Examined Publication No. SHO 41-23987. That type of chuck of the prior art, as shown in FIG. 19, comprises a chuck body 1, a chuck cylinder 2 projecting ahead from the one side of the chuck body 1 in a mated manner, and having its external peripheral surface tapered ahead, a tightening rotary sleeve 3 having an internal peripheral surface tapered ahead in correspondance with the aforesaid external peripheral surface, wherein the rotary sleeve is fitted outside the chuck cylinder, and many needle rollers 4 interposed between the cylinder 2 and the sleeve 3 so as to revolve spirally with relation to the external peripheral surface of the chuck cylinder 2.
The chuck cylinder 2, into which a cutting tool T is inserted, is structured so as to be contracted by the depression of the needle rollers 4 on the external peripheral surface, thereby gripping a cutting tool.
However this prior art has a problem; for the purpose of holding firmly a cutting tool particularly for milling which receives rather much resistance, it is necessary to increase the rigidness and intensity of the chuck cylinder by thickening the wall thereof. In doing so, the chuck cylinder 2 becomes difficult to be contracted, i.e. to grip the cutting tool securely.
In view of the above-mentioned problem of the prior chuck, the applicant of the present invention has previously filed, at the Japanese Patent Office on Mar. 19, 1982, an application with respect to another chuck preceded a chuck of the present invention, which is disclosed in Utility Model Umexamined Publication No. 58-143107 open to the public on Sept. 27, 1983. That chuck, as illustrated in FIG. 20, includes a chuck cylinder 2 extending from the side of a chuck body 1 in a mated manner, which is provided with chuck grooves 5 arranged at regular intervals consisting of axially extending slots 5a open to the internal peripheral surface of the chuck cylinder 2, and axially extending middle holes 5b communicating with said slots 5a. Other structure are identical to that of the above described prior art, and are represented by the same numerals.
In the structure of the above-mentioned chuck, the chuck cylinder 2 is provided with thinner walls 6 at regular intervals in the circumference, so that it can grip the cutting tool somewhat firmly through the contraction of the thinner walls 6 in case the thickness of the chuck cylinder 2 is a large size. Nevertherless, in the chuck having such structure, a problem remains unsolved that it is impossible to obtain a sufficient contraction amount particularly at the top end section of the chuck cylinder, i.e., it is impossible to apply force sufficient for gripping a cutting tool to the top end section. The cause of the above-mentioned disadvantage is considered as follows; In spite of the configuration of the thinner walls 6 in the top end section of the chuck cylinder 2, the top end part 6' of the cylinder 2 is not depressed directly by the needle roller 4 during the tightening operation and so the rigidity of the wall of the top end part 6' is kept. Besides, the aforesaid depression of the needle roller 4 causes the diameter of the opening of the top end part 6' to enlarge as shown in FIG. 20 by chain line, because a part of the depression force is transferred from the contracted section of the chuck cylinder 2, and absorbed in the top end part 6'. Therefore the force for gripping a tool is lowered remarkably at the top end part of the chuck cylinder.